The present invention relates to an alignment apparatus in an X-ray aligner system etc., in which a mask and a wafer have their relative positions detected in a focused state and are aligned.
A relative position detection apparatus including a dual focus optical system, for use in a proximity aligner system in which a wafer and a mask are exposed to light with a minute clearance or gap therebetween, has heretofore been known from Japanese Laid-open Patent Application No. 51-138464 or Japanese Laid-open Patent Application No. 52-126243. FIG. 1 shows a case where such known relative position detection apparatus is applied to the aligner system.
Numeral 1 in FIG. 1 designates a wafer, which is provided on its surface with a pattern 1' and three alignment marks 3 as shown in FIG. 2(A).
Numeral 2 in FIG. 1 designates a mask, which is provided on its surface with a pattern surface 2' and three alignment marks 4 as shown in FIG. 2(B).
In the dual focus detection method illustrated in FIG. 1, the alignment marks 3 of the wafer 1 and those 4 of the mask 2 are focused on a sensor 8 as will be described in detail later, whereby the relative positions of the wafer 1 and the mask 2 are detected with the images of both the alignment marks 3 and 4 superposed on the sensor 8 as shown in FIG. 2(C). Illumination light emergent from a light source 10 is projected on the mask 2 and the wafer 1 via an illuminating optical system 11, a semitransparent mirror 12, a mirror 13 and an objective 5. Light rays reflected from the wafer 1 and the mask 2 pass through the objective 5, mirror 13 and semitransparent mirror 12 again and reach a beam splitter 14, by which the light rays are split into a light path 15 and a light path 16.
Here, the light path 15 is a light path for focusing the alignment marks 4 on the mask 2 onto the sensor 8, while the light path 16 is a light path for focusing the alignment marks 3 on the wafer 1 onto the sensor 8.
The light path 16 is provided with a magnification compensating lens 17 for equalizing the magnification of the alignment marks 3 on the wafer 1 to the magnification of the alignment marks 4 of the mask 2, and a mirror 18.
The light path 15, which is the light path for focusing the alignment marks 4 on the mask 2 onto the sensor 8, is folded in order to focus the alignment marks 3 and 4 of the wafer 1 and the mask 2 on the sensor. The light beam passes through a prism 20 for compensating its optical length and further through a mirror 21, to reach a beam splitter 19.
The light paths 15 and 16 are superposed by the beam splitter 19, and the real images of the alignment marks 3 and 4 of the mask 2 and the wafer 1 having the equal magnifications are formed at a first focusing point 22.
Herefrom, the images are further passed through a relay lens 23 so as to be focused on the sensor 8.
Meanwhile, in a case where this relative position detection apparatus including the dual focus optical system is applied to a soft X-ray aligner system, a positioning accuracy of within 0.1 .mu.m is required. On the other hand, since a soft X-ray source is close to a point source, the circuit pattern on the mask can be printed on the wafer by enlarging or reducing it by changing the clearance between the wafer and the mask. By adjusting the clearance between the wafer and the mask, accordingly, the minute thermal expansion or contraction of the mask or wafer can be coped with, and the matching of patterns based on the joint use of the soft X-ray aligner system and a reduction projection aligner system (disclosed in U.S. Pat. No. 4,153,371) can be established. However, when the clearance between the mask and the wafer is adjusted as described above by moving either the mask or the wafer, the alignment marks become out of focus in the light path 15 or 16 and need to be brought into focus by moving the prism 20 etc. In a case where, in moving the prism 20 etc., this prism 20 has deviated laterally or inclined as illustrated in FIG. 3, the incident points of the light paths 15 and 16 on the sensor 8 disagree, resulting in the problem that the positioning accuracy mentioned above is not attainable.
As an expedient for solving this problem, Japanese Laid-open Patent Application No. 52-126243 discloses the use of a corner curb 20' as illustrated in FIG. 4. Here, symbol 14' denotes a semitransparent mirror, and symbols 18' and 21' denote reflectors. The expedient has the disadvantage that the corner curb 20' has three edges, which are observed and cannot be distinguished from the alignment marks of the mask and the wafer. Another disadvantage is that, since the semitransparent mirror 14' must be used, the quantities of light from the alignment marks of the mask and the wafer decrease to half, so the sensitivity lowers.